drm: xilinx: dp: Set and clear the PHY config register

There are variations of this register field between IP core versions,
but the driver is only interested in reseting the PHY. So, instead of
overwriting the default values, read-update-write the PHY reset bits.
Remove unused definition by this change.

Reported-by: Andrei-Liviu Simion <andrei.simion@xilinx.com>
Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>